1. Field of Invention
The present invention relates to a fan rotary speed controlling device and, in particular, to a fan rotary speed controlling device of a brushless DC fan.
2. Related Art
Regarding to the driving mechanism of the motor of a current brushless DC fan, a Hall sensor is involved to determine the positions of magnetic poles of the permanent magnetic rotor after the windings of the stator is powered on. Accordingly, the direction of the current flowing through the windings of the stator is changed to generate the alternative magnetic field, which can push the permanent magnetic rotor to continuously rotate. Currently, the PWM (pulse width modulation) technology is the most popular rotary speed control technology for the fan motor.
As shown in FIG. 1, a conventional fan rotary speed controlling device 1 is applied to control the rotary speed of a motor 7. The fan rotary speed controlling device 1 includes a rotary-speed voltage conversion circuit 11, a control circuit 12, a pulse width modulation (PWM) circuit 13, and a switching circuit 14. The rotary-speed voltage conversion circuit 11 receives a rotary-speed signal S1, which represents the rotary speed of the motor 7 while operating. Then, the rotary-speed voltage conversion circuit 11 converts the rotary-speed signal S1 into a voltage signal S2, which is then transmitted to the control circuit 12. The control circuit 12, which includes a PID (Proportion-Integral-Differential) controller, receives the voltage signal S2 and a target voltage signal S3, and compares the voltage signal S2 with the target voltage signal S3 to output a signal S4. The PWM circuit 13 generates a PWM signal S6 according to the signal S4 and a fundamental frequency signal S5. The switching circuit 14 turns on/off a plurality of switch elements depending on the PWM signal S6, so that it can output a control signal S7 to control the rotary speed of the motor 7.
The conventional constant-rotary-speed controlling method for motors, such as the above-mentioned PID control or programmable logic control (PLC), can achieve the desired constant-rotary-speed control for motors. However, these conventional methods can not precisely control the rotary speed of the motor, and have the problems of difficult adjustment and high cost.